(1) Field of the Invention
This invention relates to a surface flaw detection method for a metal product and an apparatus for putting the method in practice.
(2) Description of the Prior Art
For detecting the surface flaws of a metal product, various nondestructive inspections have been put into practice, one or plural kinds of methods therefor having been applied corresponding to the flaws expected to exist. For example, a magnetic inspection method for detecting leakage flux generated from the surface of an object to be inspected is mainly used to detect flaws, such as cracks, expected to extend in the regular direction to some extent and the eddy current inspection method for the flaws, such as pits, extending merely in the direction of thickness of object. The magnetic inspection method generally is (1) superior in surface flaw detection for ferromagnetic substances, such as iron and steel products, (2) capable of detecting inherent flaws even not open at the surface, and (3) capable of inspecting the position and length of the flaw on the surface, but is defective in difficulty of detecting the internal flaws. On the other hand, the eddy current inspection method is advantageous in that (1) the detection result is obtained directly by an electric output, (2) the non-contact method permits rapid inspection speeds, (3) the method is suitable for detecting surface flaws and pit flaws, (4) the scope of applicability is wide because the inspection can follow the flaws, variation in object, and dimensional change, and (5) the signal and the flaw volume have an approximately proportional relation, but is defective in that (1) the method is not applicable unless the material form is simple, (2) a flaw positioned deep under the surface is not detectable, and (3) the influence of material factor other than the object to be tested often causes noises.
Also, the magnetic inspection method is effective in magnetization perpendicular to the flaw, but it is impossible to detect flaws when the magnetization is in the same direction as the flaw because no magnetic pole is generated at the flaw and the leakage flux from the surface of object to be inspected is minute. At present, however, the following method of utilizing a plurality of magnetic fields has become possible to detect the flaws irrespective of the direction of flaw.
For example, a round steel bar 1, as shown in FIG. 1, is directly axially energized to be magnetized circumferentially and a current flows in a coil around the steel bar 1 to magnetize the bar 1 axially, so that the circumferential magnetization detects the surface flaws 1a extending circumferentially of the bar 1 and the axial magnetization detects those flaws 1b axial of the same, which has been well known.
Also, an inspection method as shown in FIG. 2 has been well known which comprises a pair of coils 2, 2 surrounding a pipe 1' and a magnet 3 having opposite magnetic poles at both sides diametrical of pipe 1', the coils 2, 2 and magnet 3 being disposed in tandem, so that the coils 2, 2 magnetize the pipe 1' axially thereof to thereby detect a circumferential surface flaw 1'b in the magnetic field by use of a magnetic field detector 2a and the magnet 3 magnetizes the same circumferentially thereof to thereby detect an axial surface flaw 1'a in the magnetic field by a magnetic field detector 3a.
The surface flaws on the metallic object, however, include the flaws called the pit flaws, as well as the cracks, the pit flaws being difficult to detect by the aforesaid magnetic detection. Hence, the detection of pit flaws, if necessary, should depend on the eddy current inspection method, the detection of crack-like flaws depending on the magnetic particle detection of high detection power. Therefore, there has been trouble such that a plurality of inspection methods are indispensable for use according to the kind of object to be inspected and to properties of the flaws.